This invention relates to an improved window structure for optical scanners, particularly the type of optical scanners having a horizontal slide having an aperture over which packages are passed through which optical scanning of the packages or articles takes place. Such scanners are now finding use in reading grocery packages having the UPC (Universal Product Code) imprinted thereon.
Such optical scaners scan the surfaces of articles of merchandise, such as groceries, for a code which can be optically read. In the grocery industry, a UPC code has now been adopted for standardizing the type of optically responsive elements incorporated on each package which will identify the product, manufacturer and quantity. As the package is moved along the checkout slide, it encounters a window formed in the slide's upper surface through which a light beam, for example a laser beam, is projected upon the package, the reflection being retrodirectively viewed and sensed. As the code on the package is scanned the reflectivity varies in intensity. Subsequently the reflective characteristic is developed into a series of digital pulses of varying width and spacing which can be decoded by a suitably programmed computer to provide both price and identification data with respect to the article. An example of the foregoing kind of equipment is shown in U.S. patent application Ser. No. 466,803 filed May 3, 1974, now abandoned, now Ser. No. 677,865 filed Apr. 19, 1976, entitled "Method and Apparatus for Reading Coding Labels", by Alfred P. Hildebrand, et al, which is assigned to the same assignee as the present application.
Such apparatus has commonly used a glass plate as a horizontal window located in the plane of the slide through which the scanning laser beam passes. The glass transmits the laser beam from optical apparatus located below the slide and serves as a window for retrodirective viewing of the article. There has been found in the course of experimental trials of such apparatus that the window suffers considerable abrasion from articles passing thereover which cause scratches. Even minute scratches tend to cause splaying of the laser beam on transmission. When scratches are sufficiently deep the integrity of the laser beam is destroyed to such a degree that reliable code reading becomes difficult, i.e., the number of erroneous readings or non-reads increases to an unacceptable level. Attempts to solve this problem directly by providing a window having a hard coated surface have been unsuccessful even though minor scratches are avoided, because certain articles which pass the window have sufficient hardness to cause a scratch in the surface coating applied to the glass. Such a scratch is unfortunately of a somewhat different character in that it tends to break and flake away portions of the coating in the adjacent area of the scratch, thus making the resultant mark far more pronounced and objectionable.
In addition, horizontally mounted windows are constantly becoming soiled because of contact with powdery and liquid substances which adhere to the window and also cause objectionable dispersion or absorption of the laser beam.